Your documentation needs some cleanup, though, because it puts some features under the Standard Markdown heading that are neither in Commonmark nor in Gruberʼs specification, e.g. strike-through and tables.
It is not really helping that your are using the same acronym, “GFM”, to refer to GitLab Flavored Markdown that everyone else is using for GitHub Flavored Markdown. You should probably also document the differences between the two, e.g. $`inline math`$, fenced block quotes >>>, videos , diffs [+ ins +] {- del -}.
We should really get on publishing extension modules with test cases, so implementations can claim interoperable conformance to them. They often differ in details, e.g. Gitlab apparently requires exactly two tildes ~~ before and after stricken text while Github collapses any matching number of tildes (like `).